One of the things growing in urban Stockholm is
nettles (Urtica dioica), which would be great for urban foraging though I just found out you probably don't want to eat those. Apparently they need a lot of nitrogen, and the reason they grow everywhere here is because of all the dogs peeing everywhere. I guess there is a big risk for contamination of some kind. Just what I heard though. Same thing goes for the ground-elder. (Aegopodium podagraria)
So I would just want to advice anyone going for urban foraging to consider what pets might have relieved themselves on your potential food.
And this is probably not the kind of "foraging" you meant, but I find myself going through the dumpsters behind
local supermarkets very often nowadays. It's surprising how fresh food you can find! This, ofcourse, is illegal (at least here in Sweden) and I don't suggest anyone else tries it (unless you have liberal dumpster-laws where you live, then I STRONGLY suggest you go for it). I even had one of the employees come out, greet me, throw more food in the dumpster, then leave. All while I was going through their trash. I guess I was lucky. =)